Rooftop Solar PV systems and corrugated tin / metal roof sheeting
On Australian rooftops, the most common alternative to a tiled roof is a sheet metal based product. These are often called names such as tin roof, corrugated iron, Colorbond, galvanised iron, galve sheet, Trim-Dek, Klip-lok, Zincalume and Galvalume.
When installing solar PV generation systems on corrugated sheet metal roofing products, consideration must be given to preserving or enhancing the existing condition of the roof.
There are four main areas that deserve the attention of a professional solar installation team:
- the interface between dissimilar metals (eg frames, supports and roof sheet), with respect to galvanic action;
- removal of swarf and kerf (drilling and cutting waste material) to prevent surface staining;
- potential for scratching or other damage to the protective coating of the sheet metal that may cause future corrosion; and
- denting and pitting of the corrogated / trapezoidal surface that may lead to pooling of water and enhanced corrosion.
Generally speaking, the installer should always take steps to avoid doing anything that may cause water to pool or collect anywhere on the roof, because water acts as an electrolyte (‘completes the circuit’, in a sense) between different metals, and even between the same metals in certain situations.
As the majority of framing materials in the solar industry are manufactured from stainless steel, aluminium or aluminium alloys, best practice dictates that a barrier (such as a UV stable resilient EPDM or Nitrile material) is placed between the aluminium feet / supports and the roof material.
The advice from Bluescope Steel regarding Zincalume Standard and Colorbond materials is as follows:
Do not allow any metals other than aluminium, galvanised steel, ZINCALUME® steel or zinc to come into direct contact with COLORBOND® steel. Any direct contact between any other metals (eg. copper and lead) and COLORBOND® steel can result in corrosion.
In this comment, the steel manufacturer represents that aluminium in contact with Zincalume will generally be ok, but in reality, for this to be true it needs to be known there are no scratches in the metal roof surface and that there will be no trapped moisture at the interface. In addition, it means that the installation process is reliant on installers correctly identifying different roof materials and adopting different approaches. For this reason, it is still considering best practice to install barriers (galvanic isolation) such as a washer or gasket layer to prevent contact between aluminium and the roof sheet.
Because ‘lead’ pencils contain lots of carbon black (and lead is no good for Zincalume anyway), it is important that pencils not be used to mark positions on roof sheeting.
After several years, these markings corrode and present as ‘knife like’ scratches – even holes – in the sheeting material.
It is preferred to use a crayon or marker pen.
Many installation kits provide stainless steel self driving screws for the attachment of the aluminium feet to the structural parts of the roof beneath the sheet through a barrier, however, these should not be in contact with the roof material. For example, stainless fasteners should not be used to repair holes or replace screws in areas of the roof where the panels will not provide protection (exposed to moisture) and without the gasket barrier of the frame.
The following datasheets and resources will help expand your knowledge of dissimilar metal action on Australian metal rooftops:
- Bluescope Technical Bulletin – Rooftop Galvanic Action of Dissimilar Metals;
- Bluescope Technical Bulletin – Flashing Materials for Colorbond and Zincalume sheet;
- Galvanic Series Chart
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